i) Field of the Invention
This invention relates to the detection and monitoring of laser radiation, as well as to devices and apparatus for such detection and monitoring.
ii) Description of Prior Art
Laser radiation is produced in transitions between energy levels in atoms and molecules as a result of stimulated emission following the interaction between light and such energy levels. For this to occur the atom or molecule must be in an excited energy level and have a vacant lower energy level. If the photon energy of incident light approximates the energy difference between the excited and lower levels, a transition is stimulated and the stored energy of the atom or molecule is emitted as laser light or radiation which has characteristics which distinguish it from light emitted by conventional sources. In particular laser radiation has a high degree of collimation, a narrow spectral line width, coherence and the ability to focus as a spot.
Laser radiation may be continuous or pulsed and may be produced in gases or solids. The properties of laser radiation have resulted in their use in a variety of applications including material processing such as welding, cutting and drilling; measurement of such parameters as velocity of fluid flow and dimensions; flaw detection and determination of surface finish. Laser radiation is employed in holography, in medical procedures and in optical-fiber communication systems.
The numerous applications of laser radiation require, in general, a precise knowledge of the laser radiation characteristics, for example, pulse height, shape, width and rise and fall times.
Devices have been developed for the detection and monitoring of laser radiation to provide the required information.
Two especially popular laser radiation sources are the CO.sub.2 (carbon dioxide) and YAG (neodymium-yttrium-aluminium-garnet) lasers, which are capable of producing pulsed or continuous infrared radiation at respectively 10.6 and 1.06 .mu. wavelength.
Due to their importance, pulsed CO.sub.2 laser detectors have been developed and studied extensively, but only three classes of detector have found wide spread use, namely, pyroelectric detectors, photon detectors and photon drag detectors.
Pyroelectric detectors employ heat transfer from the laser radiation to a pyroelectric crystal which causes a change in electric polarization, the corresponding voltage change is a measure of the laser output radiation.
Photon detectors operate on the basis of first order resistivity changes, due to electronic transitions caused by transfer of photon energy to electrons.
Photon drag detectors exploit momentum transfer electrons during interactions between photons and free charge carriers in semi-conductors.
Pyroelectric detectors are limited in their application by their relatively low intrinsic speed of response and in that the piezoelectricity of the active element produces a distorted reproduction of submicrosecond pulses.
Photon detectors are generally limited to operation at low temperatures in the range 4-30.degree. K, so as to avoid saturation problems.
Photon drag detectors have a fundamentally higher speed of response limited by the momentum scattering time. However, the responsitivity is several orders of magnitude less than that of pyroelectric detectors.